For the record, THIS is not an acceptable power steering fluid (my mistake early on in my BMW life)!

While I'm at it, Doru suggested today we modify our habits to include a power steering fluid reservoir replacement with every oil change, and I concur (here is my more recent attempt at replacing my power steering fluid, which was about half a quart of Dexron VI as Dexron III is no longer legally certified by GM).

Also, if you're leaking fluid on the oil cap, I suggest you replace the o-ring whenever you replace your two lower oil filter housing o-rings (every few years) as I show below:

And, every few years, check and repair your leaking under reservoir hose as cn90 shows below: